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Einstein's 'Mirror' Thought Experiment and the Speed of Light Question
Kapellmeister
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I just don't understand it so can someone have a go at explaining?
Einstein had a thought experiment in which he imagined he was travelling at the speed of light while hold a mirror stretched out in front of him (we know it's not actually possible to do but it's just a thought experiment so go with it).
He was trying to work out whether he would still see his reflection in the mirror. He decided that it was possible (because if he couldn't then it would mean he would know that he was in motion, or something like that).
But surely if he was able to see his face in the mirror then it would mean that the photons leaving his face would've had to have been travelling faster than the speed of light in order to get to the mirror's surface in the first place.
I've looked online and I can't see an actual explanation of why this is possible.
We know the speed of light is constant for all observers. But if he was travelling at the speed of light then how can the photons leave his face faster than the speed of light in order to reach the mirror's surface?
I think I'm missing something really obvious but I just don't get it.
Einstein had a thought experiment in which he imagined he was travelling at the speed of light while hold a mirror stretched out in front of him (we know it's not actually possible to do but it's just a thought experiment so go with it).
He was trying to work out whether he would still see his reflection in the mirror. He decided that it was possible (because if he couldn't then it would mean he would know that he was in motion, or something like that).
But surely if he was able to see his face in the mirror then it would mean that the photons leaving his face would've had to have been travelling faster than the speed of light in order to get to the mirror's surface in the first place.
I've looked online and I can't see an actual explanation of why this is possible.
We know the speed of light is constant for all observers. But if he was travelling at the speed of light then how can the photons leave his face faster than the speed of light in order to reach the mirror's surface?
I think I'm missing something really obvious but I just don't get it.
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I don't know. Maybe it's a bit of a crap experiment. From what I can gather, Einstein wanted to know whether anyone travelling at light speed would be aware that they were moving.
Imagine being in an enclosed box, with a light bulb for company, and that you're travelling along at light speed. Theoretically, you wouldn't be able to know that you're even in motion at all, let alone travelling so fast. But if you couldn't see your face then you would know that you're in motion, and this overturns all sorts of notions of relativity that Galileo concocted in the 17th century. So Einstein concluded that you must be able to see your face or Galileo was wrong.
edit........by which I mean 'if the enclosed box were stationary'
I did wonder whether travelling at light speed means that light still acts as if you were actually stationary. Then the speed of light would retain its constant speed for all observers, as it's supposed to. So even travelling at light speed means that light disappears from you at the speed of light. But I can't understand how that could work.
Yes, it's an identical scenario really. But if what you say is correct, how does something travelling at light speed move away from something already travelling at light speed? Is it because of time dilation?
I expect you would see nothing at all, since you would be everywhere and nowhere at the same tme. I wonder how God manages it.?
I sometimes wonder if we are supposed to prefix all these statements with 'It is as if .....'
'Its is as if light travels at a constant spread for all observers'
'It is as if light behaves both as a wave and a particle'
Meaning that we have no idea what the underlying reality is that is manifesting these 'It is as if ...' statements.
hi ho silver:cool:
Well that's that then.
Let's instead say you are travelling at some fraction of the speed of light that is possible. Now look in your mirror - does the light travel to the mirror at some speed less than that fraction (or maybe faster) you are travelling at? The answers is no. You as an observer would see it travelling at the correct speed. What does an external observer see therefore?
Well as I understand it, an external observer would see you moving through time slower and therefore the light would still be travelling at the speed of light.
ie. it is relative to the observer and time adjusts to keep the observer seeing the light at the correct speed.
Something like that anyhow. I'm sure cleverer folks will come along to describe it properly.
Edit: and here is that cleverer person: https://www.youtube.com/watch?v=n2s1-RHuljo
No, everything travelling with you would appear as normal.
You could spend a year (or any period of time) at that speed doing a years worth of activities and becoming a year older in the process, but when you slowed down again and rejoined the rest of us slowcoaches we'd think you hadn't been gone for any time at all.
That's all assuming you could actually attain the speed of light, do so in a reasonable period of time and somehow end up in the place where you started.